It is well known to produce maleic anhydride by subjecting a hydrocarbon to catalytic oxidation in a gas phase. Heretofore, the production of maleic anhydride has been accomplished by the reaction of benzene and air as raw materials in the presence of a vanadium pentoxide-based catalyst. In recent years, processes involving the use of a straight-chain hydrocarbon having four carbon atoms such as butane, butene and butadiene have been developed. Among these processes, one involving the reaction of n-butane, which is a saturated hydrocarbon, as a raw material in the presence of a catalyst comprising a vanadium-phosphorus mixed oxide as an active component has been mainly employed. As the active component to be incorporated in such a catalyst, divanadyl pyrophosphate ((VO).sub.2 P.sub.2 O.sub.7) has been known to exhibit excellent performance. Many references concerning this compound have been published (e.g., Chem. Rev. 88, p. 55-80 (1988)).
The foregoing reaction is effected in a fluidized bed process or a fixed bed process. In some detail, a hydrocarbon and an oxygen-containing gas, normally air, are fed as raw material into a reactor in such a manner that the concentration of the hydrocarbon reaches from about 1.5 to 10%. The reaction mixture is then allowed to undergo reaction at a temperature of from 300.degree. C. to 600.degree. C. The reaction gas coming out of the reactor contains maleic anhydride as well as carbon monoxide, carbon dioxide, water and other reaction products. The separation and recovery of maleic anhydride from the reaction gas is accomplished by a process which comprises cooling the reaction gas to condense maleic anhydride, a process which comprises allowing the reaction gas to come in contact with water so that maleic anhydride is collected as maleic acid in water, a process which comprises allowing the reaction gas to come in contact with an organic solvent such as phthalic acid ester or alkyl ester of hydrogenated phthalic acid so that maleic anhydride is collected in the organic solvent.
In the commercially practiced process for the production of maleic anhydride, the hydrocarbon conversion in the reactor [number of mole of hydrocarbon consumed in the reaction per pass/number of mols of hydrocarbon supplied into the reactor.times.100 (mol %)] is kept as high as possible. This is required to minimize the amount of hydrocarbon as a raw material required to produce maleic anhydride. In general, the hydrocarbon left unreacted in the reactor is incinerated in a waste gas burning apparatus.
On the other hand, it has been known to reduce the hydrocarbon conversion, making it possible to reduce the proportion of carbon monoxide or carbon dioxide to be produced as a by-product and hence enhance the maleic anhydride selectivity [number of mols of maleic anhydride produced by the reaction/number of mols of hydrocarbon consumed in the reaction.times.100 (mol %)]. Accordingly, if the hydrocarbon conversion can be kept low and the hydrocarbon left unreacted can be recovered and again supplied for reaction as a raw material, the unreacted hydrocarbon which would otherwise be incinerated and the hydrocarbon which would otherwise be converted to carbon monoxide or carbon dioxide can be partly converted to maleic anhydride, making it possible to drastically reduce the amount of hydrocarbon to be consumed as a raw material in the production of a unit amount of maleic anhydride. Therefore, this process is an extremely fascinating on an economical basis.
The foregoing process is also advantageous in that the recovery of the unreacted hydrocarbon which would be otherwise incinerated makes it possible to drastically reduce the amount of gas to be wasted during the production of maleic anhydride, particularly the emission of carbon dioxide, which is one of the greenhouse effect gases the emission of which has recently faced a growing demand for reduction, and hence drastically reduce the influence on the environment.
In practice, JP-A-49-81314 (The term "JP-A" as used herein means an "unexamined published Japanese patent application"), JP-A-54-151910 and JP-A-59-29679 propose a process which comprises reducing the hydrocarbon conversion in the reactor to keep the maleic anhydride selectivity high while the unreacted hydrocarbon is being partly recovered and returned to the reactor.
However, none of these proposals have ever been commercially practiced. This is because the hydrocarbon concentration needs to be higher than ever to prevent the drop of the productivity of maleic anhydride while keeping the conversion in the reactor low. If the hydrocarbon concentration is higher than ever, high temperature portions called "hot spot" occur in the reactor, causing degradation of catalyst. This is also because when the unreacted hydrocarbon is recovered, carbon monoxide or carbon dioxide produced as by-product, too, is recovered, making it necessary to use large amount of pure oxygen or oxygen enriched air, which is an expensive oxygen source, due to restrictions on material balance.
Further, economically favorable reaction conditions differ greatly from that of the conventional once through reaction. Thus, the criteria of explosion safety of the feed gas to, or the effluent gas from the reactor, product recovering apparatus or hydrocarbon recovering apparatus greatly differ. The foregoing proposals contain reference to the safety of the reactor feed gas but have no reference to the safety of the entire recycle process.
On the other hand, JP-A-1-165564 proposes a process which comprises returning unreacted hydrocarbon recovered by an apparatus for selectively separating hydrocarbon to a reactor wherein the content of flame suppressor is regulated to prevent a mixture of hydrocarbon and oxygen from producing a flammable mixture. However, this proposal regulates the safety of the stream from the reaction apparatus to the hydrocarbon recovering apparatus and back to the reaction apparatus but doesn't suffice for the safety of the entire process for the production of maleic anhydride. In other words, it is substantially difficult to completely recover hydrocarbon by the hydrocarbon recovering apparatus. Thus, the exhaust gas after recovering hydrocarbon is a mixed gas containing flammable gases such as hydrocarbon and carbon monoxide and oxygen. Accordingly, the explosion safety of the mixed gas must be considered.
In accordance with economically favorable conditions under which a high productivity can be realized, that is, the concentration of maleic anhydride in the reaction gas can reach not less than 2 vol %, the concentration of carbon monoxide in the reaction gas, too, is higher than under the conventional conditions. Accordingly, the exhaust gas after recovering hydrocarbon has a higher carbon monoxide concentration than the conventional composition the safety of which has heretofore been known. Thus, it is likely that the explosive region of the exhaust gas is expected to be wider. Nevertheless, no specific methods for controlling the explosion safety have been known.
In other words, some methods have been proposed which comprise recovering and recycling unreacted hydrocarbon to the rector while keeping the hydrocarbon conversion in the reactor low to enhance the maleic anhydride selectively for the purpose of efficiently producing maleic anhydride. However, all the foregoing proposals are disadvantageous in that the use of pure oxygen or oxygen enriched air, which is expensive, adds to the production cost and the enhancement of productivity is accompanied by the generation of enormous heat that deteriorates the performance of the catalyst. These proposals are also disadvantageous in respect to safety control. Thus, these proposals are not necessarily excellent methods. In actuality, these proposals have never been commercially practiced.